JPH09232375A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To absorb the warp of a printed wiring board, enable obtaining connection of high reliability, and obtain a semiconductor device of low cost, by arranging a plurality of solder connecting members wherein tin having a spherical outline and specified surface tension is used for main material, to which lead is added. SOLUTION: A plurality of solder connecting members 1 which protrude from the surface of a board 4 and have a spherical outline are arranged. In the solder connecting members 1, tin which is fused again while applying self weight and has surface tension maintaining the height at least 75% of the original height of the outline is used as main material, to which lead is added. The loadings of the lead is preferably set to be 5-20%. For example, solder material turning to a solder bump is composed of Sn as main material and Pb whose content is 10%. A solder ball (sphere whose diameter is 0.67mm) is composed of eutectic solder (Pb 37%, Sn 63%). Then the solder material and a semiconductor device having the solder ball are prepared. Solder paste is spread on pads arranged on a printed wiring board 2, a board 4 of the semiconductor device is positioned on the pads and mounted, and reflow is performed. Thereby the sinking of a solder ball 1 whose main material is Sn is little, and its ratio is about 10%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device having a plurality of solder connection members (hereinafter simply referred to as solder bumps) projecting from a substrate and having a spherical outline.

[0002]

2. Description of the Related Art As a semiconductor device of this type having a large number of solder bumps arranged vertically and horizontally on a substrate surface, for example,
A semiconductor device whose package is called a ball grid array is given. In this ball grid array, the size of the package, the number of solder bumps, and the pitch have been reduced with the increase in the number of elements forming the electronic circuit and the increase in the density of the elements.

[0003] The connection between the solder bumps of the large package and the conductive pads of the printed wiring board is usually performed by a method called face-down. In this method, for example, a printed wiring board is placed on a stage of an infrared heating furnace, the solder bumps are turned downward, and the conductive bumps are brought into contact with the conductive pads of each printed wiring board, and the package is placed on the printed wiring board. Is a method of melting and connecting.

Conventionally, a eutectic solder containing 63% of tin in lead, which is a main material, has been used as a material of the solder bump. The advantage of the solder bump made of the eutectic solder is that the relaxation at the time of reflow is large and the outer height can be maintained only about 60% of the original height, but even if the printed wiring board is warped, the melted portion is warped. Thus, coplanarity (flatness) of the printed wiring board can be obtained, and the self-alignment function works due to the surface tension of the melted solder, so that the connection can be performed without any positional displacement.

[0005] However, the reliability of the connection portion to the printed wiring board has become a problem with the height of the outer contour of the bump made of the eutectic solder. This problem is caused by the fact that the stress acting on the connection portion of the bump due to the difference in thermal expansion between the substrate and the printed wiring board becomes large due to the low height of the bump, and this stress is repeated every time the power is supplied to and interrupted from the IO port. In addition, the connection of the bump to the printed wiring board will eventually fatigue and shear.

The stress σ acting on the connection interface
Is, as is well known, σ = ΔαΔTG1 / h. Here, Δα is the difference between the coefficients of thermal expansion, ΔT is the temperature difference that changes, G is the elastic coefficient, l is the distance from the center of the package to the solder bump to which stress is applied, and h is the height of the solder bump after connection. . The equation for calculating this stress is established assuming that the pillar-shaped solder bumps after reflow do not elastically deform.However, if the height of the solder bumps during reflow can be maintained high, the stress decreases and fatigue fracture occurs. Difficult to say.

Therefore, various attempts have been made in recent years to obtain the height of the solder bump. For example, IEEE T
RANSACTIONS ON COMPONENT,
PA-CKAGING, AND MANUFACTUR
ING TECHNOLOG-YPARTB, VOL1
8, NO1, FEBURUARY 1995, pages 53-56. This solder bump is made of lead 9
In this structure, a high melting point solder of 0% and tin 10% is used as a core, and a relatively low melting point eutectic solder is used for connection between a printed wiring board and a substrate. According to this solder bump,
At the time of connection by reflow, core dissolution is small and standoff (bump height after connection) can be obtained.

As another example, as disclosed in US Pat. No. 5,147,084, the purpose is to make it easy to repair the object, or to avoid contact or bridging of an adjacent connection part after connection. The high melting point solder ball is placed on the low melting point solder of the board, and the surrounding area is surrounded by epoxy resin. During the reflow, the low melting point solder of the connected board is connected to this high melting point solder ball, and the height of the solder ball is maintained. And a structure surrounding the connection portion with epoxy resin.

[0009]

The above-mentioned connection portion formed of a high melting point solder core or solder ball can secure a height to reduce shear stress, but has a low melting point to absorb warpage of a printed wiring board. Requires the thickness of the solder layer. It is difficult to form such a thick and hundreds of solder layer patterns without breaking their shapes even by screen printing using a mask which is an inexpensive method.

[0010] In the latter case of a solder ball, if an epoxy resin as an insulating material is applied to a substrate in advance and a hole is formed by photolithography, and the hole is filled with a low melting point solder, the solder described above will be used. Deformation of the layer pattern may be eliminated. However, the number of steps of coating a resin to a predetermined thickness and making holes is increased, and it is unavoidable that the increase in the number of steps decreases the yield and increases the cost.

Further, when two solder portions having different compositions are reflowed and connected as described above, foreign substances remain due to insufficient cleaning, and bubbles are generated on the joint surface with a rise in temperature. There is a concern that the connection strength is reduced.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an inexpensive semiconductor device capable of absorbing a warp of a printed wiring board and obtaining a highly reliable connection.

[0013]

SUMMARY OF THE INVENTION A feature of the present invention is that it has a spherical outer shell protruding from the substrate surface and is re-melted while applying its own weight to maintain the outer shell at its original height of at least 75%. A semiconductor device comprising a plurality of solder connection members containing tin as a main material and adding lead. Preferably, the lead contains 5 to 20%.

[0014]

Next, the present invention will be described with reference to the drawings.

FIGS. 1 (a) and 1 (b) are graphs showing a change in surface tension depending on the composition ratio of tin and lead, and solder bumps formed by solder balls, for explaining an embodiment of the semiconductor device of the present invention. FIG. Generally, an alloy of lead and tin is used as a material for the solder bump. Depending on the content ratio of lead and tin, the melting point differs and the mechanical properties also differ. For example, "Handbook o" published in 1989
f Printed Circuit Design discloses that the surface tension changes depending on the content of lead and tin. That is, as the content of tin (Sn) with respect to lead (Pb) increases, the surface tension increases as shown in FIG.

The present invention has been made on the assumption that the height of the solder bumps can be increased by the surface tension when reflowing by increasing the content of tin in the solder and increasing the surface tension. As a trial, a semiconductor device was prepared in which a solder material to be a solder bump was mainly Sn and a solder ball (spherical having a diameter of 0.67 mm) with a Pb content of 10% and a eutectic solder (Pb 37%, Sn 63%) was attached. Then, as shown in FIG. 1 (b), a solder paste is applied to the pads provided on the printed wiring board 2, the substrate 4 of the semiconductor device is positioned and mounted thereon, and reflowed. Sinking of 1 is small and 10%
It was about. On the other hand, solder ball 1 made of eutectic solder
Had a large sinking height of only about 60%.

Further, unlike the eutectic solder ball, it does not sink as much as 40%, and only needs to sink by about 10%. Therefore, the horizontal spreading of the drum-shaped solder ball depends on the diameter of the pad and the solder ball 1. If it is considered to be equal to the height of the solder ball, even if it is simply calculated geometrically, it spreads only 5% in the lateral direction, and adjacent solder balls do not come into contact with each other. This means
It can be said that there is no need to provide an insulator between the solder balls and the pitch can be further reduced.

FIG. 2 is a table showing the results of a temperature cycle test of the formed connection. Incidentally, a plurality (15) of these two types of samples were subjected to a temperature cycle test (−40 °).
C / + 125 ° C., 1 CYCLE / Hr), as shown in FIG. 2, three samples of the eutectic solder had a breakage of the connection part in about 500 cycles, whereas 90 In the case of the solder containing% tin, none of the samples broke even after 1000 cycles. In addition, since the reliability of the connection is improved and tin is mainly used, the thermal conductivity and the electrical conductivity are also improved, and heat dissipation from the semiconductor pellet is required. Obtained advantageous findings.

FIG. 3 is a graph showing the relationship between the height that can be maintained at the surface tension when different loads are applied during reflow and the lead content in tin. Here, the magnitude of the load (P) applied to one solder bump at the time of reflow in this type of semiconductor device is examined, and in parallel with this, various solder balls having different lead contents are prepared using tin as a main raw material. Each of the solder balls was adhered to pads of boards having different weights and the number of pads, face down to a printed wiring board, reflowed, and the height (H) at which the solder balls could be maintained was measured. As a result, a graph as shown in FIG. 3 was obtained.

FIGS. 4 (a) and 4 (b) are views showing a process sequence for explaining a process of connecting the semiconductor device of the present invention to a printed wiring board. Next, connection of the ball grid array, which is the semiconductor device, to a printed wiring board will be described. First, a printed wiring board to be connected to a semiconductor device is prepared. The warpage of the printed wiring board thus prepared is 0.05 to 0.12 in a 40 mm square area.
mm. In this measurement, a printed wiring board was placed on a precision platen, and a stylus was brought into contact with each point of the printed wiring board with a dial gauge, and the maximum reading of the dial gauge pointer at that time was defined as warpage (δ). .

Next, as shown in FIG. 4A, a ball grid array in which a solder paste is applied to the pads of the substrate 4 on which the heat slugs 3 are attached and the solder balls 1 are placed thereon is previously prepared. The solder paste is thinly applied to the pad 5 and positioned on the printed wiring board 2 having the warpage δ.
Next, the periphery of the ball grid array was heated by, for example, about 220 ° C. in an infrared furnace, and the solder balls were completely re-melted, and the ball grid array was pulled out of the infrared furnace and cooled. As a result, as shown in FIG. 4B, the solder ball is formed into a solder ball 1a having a sinking and drum-shaped height, and the ball grid array is connected to the printed wiring board 2 via the solder ball 1a. More than half of the warpage (δ) of the printed wiring board 2 before reflow was absorbed.

FIG. 5 shows the results of the absorption of the warpage of the substrate and the connection reliability when solder balls having different lead contents are connected to ball grid arrays having various weights and different numbers of pins. It is a table. Here, the connection between the ball grid array having a different load per solder ball and the printed wiring board having a distortion of about 0.1 mm was performed by using the above-described connection method with the solder balls having different compositions (diameter: 0.67 mm). ).

As a result, as shown in the table of FIG. 5, in the ball grid array (1A to 1D) having a light weight,
Even with the solder balls of tin 80 and lead 20, the solder ball at the center of the package sinks greatly and absorbs the warpage of the substrate, and no breakage of the connection portion was observed until 500 cycles in the temperature cycle.

Further, in the ball grid array having its own weight slightly heavier than the above-mentioned ball grid array, the lead content is 15%.
The following were good, and the solder ball having a lead content of about 10% was good even for the ball grid array having the highest weight.

From this table, it can be said that when a solder ball having a height of 0.67 mm reflows while applying its own weight, the solder ball has a certain height due to its surface tension.
If 52 mm can be maintained, a desired connection reliability can be obtained. That is, 0.52 / 0.67 of the original height
It suffices to select a solder material having a lead content of tin so as to maintain × 100 = 77% or more. In the case of a ball grid array having a light weight per pin, a tin content of 20% or less should be selected for tin, and a lead content of about 10% should be selected for the heaviest one.

Also, as shown in the graph of FIG.
Given the use of solder balls such as 95% tin and 5% lead, heavy radiators, which were conventionally mounted later,
The height of the solder bumps can be maintained even if the solder bumps are attached first and reflowed. This has the advantage that the mounting process on the printed wiring board becomes simpler. However, in this case, since the heat capacity of the radiator increases, it is desirable to use a vapor phase soldering method instead of a hot air infrared furnace when reflowing.

[0027]

As described above, according to the present invention, the composition of a spherical solder connecting member projecting from the substrate surface of a semiconductor device and connecting to a pad of a printed wiring board is composed mainly of tin and at most 20% of lead is added. When the connection member is re-melted, the surface tension is increased so that the height can be maintained high, and the connection member can be maintained high without providing a means for maintaining the height particularly high. This has the effect of reducing the thermal stress applied to the wire and improving the reliability without shearing the connection portion even when the thermal stress is applied repeatedly.

In addition, since the connecting members can be kept high, the spread in the lateral direction can be reduced, and there is no need for an insulator for preventing contact between adjacent connecting members.
There is an effect that the cost can be greatly reduced without increasing the number of steps due to the provision thereof and the yield reduction due to the increase in the number of steps.

[Brief description of drawings]

FIG. 1 is a graph illustrating a change in surface tension depending on a composition ratio of tin and lead and a diagram illustrating solder bumps formed by solder balls for describing an embodiment of a semiconductor device of the present invention.

FIG. 2 is a table showing a result of a temperature cycle test of a formed connection portion.

FIG. 3 is a graph showing the relationship between the height that can be maintained at the surface tension when different loads are applied during reflow and the lead content in tin.

FIG. 4 is a diagram illustrating a process sequence for explaining a process of connecting a semiconductor device of the present invention to a printed wiring board.

FIG. 5 is a table showing the results of strain absorption and connection reliability when solder balls having different lead contents are connected to a printed wiring board for various ball grid arrays having different weights and different numbers of pins.

[Explanation of symbols]

 1, 1a solder ball 2 printed wiring board 3 heat slug 4 board 5 pad

Claims (2)

[Claims] 1. A main material comprising tin, which has a spherical outer shape projecting from the substrate surface and has a surface tension maintaining at least 75% of the original height of the outer shape by remelting while applying its own weight. A semiconductor device comprising a plurality of solder connection members. 2. The semiconductor device according to claim 1, wherein the lead content is 5 to 20%.